This invention relates to silicone compositions having an improved heat transfer ability, and semiconductor devices using the same.
IC packages such as central processing units (CPU) and other electronic parts mounted on printed circuit boards will deteriorate their performance or even fail on account of temperature rises by the heat generated during operation. In the prior art, heat dissipating sheets or grease having a good heat transfer ability is employed between the IC package and the finned heat sink. The heat dissipating sheets have the advantage of easy mounting. Since the surface of CPU or finned heat sink is microscopically irregular despite apparent flatness, it is difficult in practice to intimately bond the heat dissipating sheet to the adherend surface, often leaving an air gap therebetween. This leads to the disadvantage that the heat dissipating sheet fails to exert the heat dissipating effect as desired. One typical solution proposed thus far is to provide the heat dissipating sheet with a pressure-sensitive adhesive layer for achieving a more intimate bond, which is still insufficient. The heat dissipating grease can intimately follow and contact the surface of CPU and finned heat sink independent of irregularities on the adherend surface, but can foul adjacent parts and gives rise to the problem of oil leakage during long-term service. To overcome these problems, JP-A 61-157569 and 8-208993 propose the use of liquid silicone rubber compositions as potting agent or adhesive. However, these silicone rubber compositions have a short thermal conductivity on account of the reduced content of heat conductive filler. Due to heat release from CPU and moisture in the ambient atmosphere, the silicone rubber compositions in the cured state tend to gradually harden, finally losing flexibility so that they peel off from the substrate or CPU. As a result, the compositions undesirably increase their thermal resistance with the lapse of time.
An object of the invention is to provide a heat conductive silicone composition which has a high thermal conductivity and maintains flexibility even when exposed to heat over a long period of time. Another object is to provide a semiconductor device using the same.
The invention is directed at a silicone composition of the addition reaction curing type comprising an organopolysiloxane having at least two alkenyl groups in a molecule and an organohydrogenpolysiloxane having at least two silicon atom-bonded hydrogen atoms in a molecule. It has been found that by using specific amounts of organohydrogenpolysiloxanes of the following general formulae (1) and (2) in combination, blending a mixture of aluminum powder and zinc oxide powder in a weight ratio of from 1/1 to 10/1 as a filler, and optionally, blending a long chain alkyl group-bearing organosilane of the following general formula (3), there is obtained a heat conductive silicone composition which has a fully high thermal conductivity by virtue of a possible increase in the amount of filler blended and does not lose flexibility even when exposed to heat for a long period of time. Efficient heat dissipation is achieved by interposing a cured film of the heat conductive silicone composition between a semiconductor chip and a heat dissipator. More specifically, in a heat dissipating means for IC package comprising an IC package mounted on a printed circuit board and a heat dissipator disposed on the surface of the IC package, the heat conductive silicone composition is cast between the IC package and the heat dissipator and heat cured thereat to form a cured film having a thickness of 10 to 100 xcexcm.
Accordingly, the invention provides a heat conductive silicone composition comprising
(A) 100 parts by weight of an organopolysiloxane containing at least two alkenyl groups in a molecule and having a viscosity of 10 to 100,000 mm2/s at 25xc2x0 C.,
(B) an organohydrogenpolysiloxane of the general formula (1): 
wherein R1 is an alkyl group of 1 to 6 carbon atoms, n and m are positive numbers satisfying 0.01xe2x89xa6n/(n+m)xe2x89xa60.3,
(C) an organohydrogenpolysiloxane of the general formula (2): 
wherein R2 is an alkyl group of 1 to 6 carbon atoms, and k is a positive number of 5 to 1,000,
components (B) and (C) being contained in such amounts that the ratio of the number of Sixe2x80x94H groups in components (B) and (C) combined to the number of alkenyl groups in component (A) is from 0.6/1 to 1.5/1, and in such a proportion that the ratio of the number of Sixe2x80x94H groups in component (C) to the number of Sixe2x80x94H groups in component (B) is from 1.0/1 to 10.0/1,
(D) 800 to 1,200 parts by weight of a filler consisting of aluminum powder having a mean particle size of 0.1 to 50 xcexcm and zinc oxide powder having a mean particle size of 0.1 to 5 xcexcm in a weight ratio of from 1/1 to 10/1,
(E) a catalyst selected from the group consisting of platinum and platinum compounds, in such an amount as to give 0.1 to 500 ppm of elemental platinum based on the weight of component (A), and
(F) 0.01 to 1 part by weight of a regulator for suppressing the catalytic activity of component (E). The composition has a viscosity of 50 to 1,000 Paxc2x7s at 25xc2x0 C.
In a preferred embodiment, the composition further includes (G) 0.01 to 10 parts by weight of an organosilane of the general formula (3):
R3aR4bSi(OR5)4-a-bxe2x80x83xe2x80x83(3) 
wherein R3 is an alkyl group of 6 to 15 carbon atoms, R4 is a monovalent hydrocarbon group of 1 to 8 carbon atoms, R5 is an alkyl group of 1 to 6 carbon atoms, xe2x80x9caxe2x80x9d is an integer of 1 to 3, xe2x80x9cbxe2x80x9d is an integer of 0 to 2, and a+b is an integer of 1 to 3.
Also contemplated herein is a semiconductor device comprising an IC package mounted on a printed circuit board and a heat dissipator disposed on the surface of the IC package for dissipating heat from the IC package, a cured film of the heat conductive silicone composition defined above having a thickness of 10 to 100 xcexcm being interposed between the IC package and the heat dissipator.